1. Field of the Invention
The present invention relates to a magnetic field detecting element which is used in a hard disk drive, and particularly to a method for manufacturing a magnetic field detecting element.
2. Description of the Related Art
A spin-valve type GMR (Giant Magneto-Resistive) head is known as a magnetic head which meets requirements for high sensitivity and high output. A magnetic field detecting element that is used in a spin-valve type GMR head has a free layer and a pinned layer. These layers are stacked with a non-magnetic spacer layer interposed therebetween. The free layer is made of a ferromagnetic material, and the magnetization direction changes in accordance with an external magnetic field. The pinned layer is made of a ferromagnetic material, and the magnetization direction is fixed with respect to the external magnetic field. The magnetization direction of the free layer forms a relative angle relative to the magnetization direction of the pinned layer in accordance with the external magnetic field, and spin dependent scattering of conduction electrons varies in accordance with the relative angle so as to produce a change in magneto-resistance. A magnetic head detects the change in magneto-resistance, and reads magnetic information on a recording medium.
The pinned layer may be constructed as a so-called synthetic pinned layer. A synthetic pinned layer has an outer pinned layer whose magnetization direction is fixed with respect to an external magnetic field, an inner pinned layer which is disposed closer to the spacer layer than the outer pinned layer, and a non-magnetic intermediate layer sandwiched between the outer pinned layer and inner pinned layer. The magnetization direction of the inner pinned layer is firmly fixed due to anti-ferromagnetic coupling with the outer pinned layer. Further, since magnetic moments of the outer pinned layer and inner pinned layer cancel each other out, leakage of magnetic field is limited as a whole.
Among many types of GMR heads, a CPP (Current Perpendicular to the Plane) GMR head, in which sense current flows in a direction perpendicular to layer surfaces, has been investigated recently because of the potential of higher output. A CPP-GMR head is also advantageous in that improved heat dissipation efficiency, which is caused by the arrangement in which the magnetic field detecting element is coupled to shield layers via metal layers, allows large operating current. In a CPP-GMR head, magneto-resistance, as well as electric resistance, is increased in accordance with a reduction in the cross-sectional area of the magnetic field detecting element. In other words, a CPP-GMR head has a further advantage that it is more suitable for a narrow track width.
The change in magneto-resistance is increased in accordance with an increase in the spin polarizability of the free layer and pinned layer. Accordingly, a free layer and a pinned layer which are made of materials having large spin polarizabilities enable a further increase in magneto-resistance (MR ratio) and in output. A magnetic material having a spin polarizability of 100% or approximately 100% is called half metal. A Heusler alloy is known as a material which realizes the half metal. In recent years, it has been proposed to use the Heusler alloy for the free layer and pinned layer, instead of a CoFe alloy or a NiFe alloy which have conventionally been used. For example, Japanese Patent Laid-Open Publication No. 2003-218428 discloses a technique for using Co2MnZ (Z is an element selected from the group consisting of Al, Si, Ga, Ge, Sn) for a magnetic field detecting element in a CPP-GMR head. Japanese Patent Laid-Open Publication No. 2004-221526 discloses a technique for using Co2 (FexCr1-x)Al for a magnetic field detecting element in a TMR (Tunnel Magneto-Resistive) head and in a CPP-GMR head.
Although a CPP-GMR head having the Heusler alloy exhibits a relatively large MR ratio due to high polarizability, as mentioned above, it has the disadvantage that it has large magneto striction and is unstable as a magnetic field detecting element when the Heusler alloy is used for a free layer. Specifically, an end surface of a magnetic head is exposed to a recording medium that is opposite to the magnetic head under a stress state that is induced by other layers, such as the over-coat layer. Therefore, the symmetry of stress is lost, and tension stress is generated in the element along a height direction, which is a direction that is perpendicular to the air bearing surface. For this reason, if magneto striction is increased in the free layer, then magnetic elastic energy is increased, and large anisotropy is induced in the free layer along the height direction of the element. The free layer generally responds linearly about the track width direction. However, if the free layer exhibits large magneto striction and resultant large anisotropy along the height direction of the element, then the linear response is impeded, leading to unstable operation as an element. For this reason, magneto striction is preferably limited to no more than approximately +3×10−6. However, the Heusler alloy often exhibits a magneto striction that exceeds +10×10−6.